1. Field of the Invention
The present invention is related to the field of electric wireline well logging instruments used to measure porosity of an earth formation. More specifically, the present invention is related to instruments used to measure formation porosity by making measurements of epithermal neutron activity.
2. Description of the Related Art
Wellbores are drilled through earth formations for the purpose, among others, of extracting oil and gas. If present in a particular earth formation, the oil and gas typically exist within voids, or pore spaces in the formation. Various types of instruments have been devised to make measurements from within the wellbore of the fractional formation volume occupied by the pore spaces and the fluid content of those pore spaces within the particular earth formation.
The various instruments are typically lowered into the wellbore at one end of an armored electrical cable. Sensors in the instruments make various measurements and transmit signals to recording equipment located at the earth's surface, where the signals can be decoded and convened into the measurements of interest.
One type of instrument known in the an for measuring the fractional volume of pore space is an epithermal neutron porosity tool. The epithermal neutron porosity tool typically includes an externally controllable, pulsed source of high energy, or fast, neutrons and one or more neutron detectors which can be selectively sensitized to neutrons which have dropped in energy content to the so-called epithermal level. Fast neutrons emanating from the source can collide with atomic nuclei in the earth formation. At each collision, some of the energy of the fast neutrons can be lost by transfer of momentum to the colliding nuclei. The fast neutrons are slowed by these collisions until they drop in energy to the epithermal, and then the thermal level, whereupon some of the neutrons can be absorbed by certain materials in the earth formation, such as chlorine, which have a propensity to absorb thermal neutrons.
Transfer of neutron momentum is most efficient, and therefore occurs in the shortest time and distance from the source, when neutrons collide with subatomic particles of substantially the same mass as the neutron. In earth formations, subatomic particles close in mass to neutrons typically are hydrogen nuclei present in the fluids, which can exist in the pore spaces. Higher fractional pore volume in a particular formation, and the associated higher fluid content per unit volume of that formation, typically results in shorter slowing-down-length and shorter die-away time due to the higher concentration of hydrogen nuclei.
Some epithermal neutron tools known in the art can make measurements corresponding to the amount of time taken for the neutrons to slow down to the epithermal energy level, these tools making measurements in order to determine a so-called "die-away" rate. Other tools known in the art can make measurements corresponding to the number of epithermal neutrons at various distances from the source, these tools being so-called "slowing-down-length" measuring instruments. Still other tools known in the art can combine die-away and slowing-down-length measurements in order to determine the fractional volume of pore space, which measurements are corrected for certain effects of the wellbore environment.
An epithermal neutron die-away measurement tool known in the art is described, for example in U.S. Pat. No. 5,345,077 issued to Allen et al. The tool in the Allen et al '077 patent includes a pulsed source of high energy neutrons and a detector mounted in a pad on an extensible arm, the detector being adapted to measure epithermal neutrons at a plurality of time intervals from the time the source is "pulsed" to irradiate the formation with a "burst" of fast neutrons.
The detector measurements in the tool disclosed in the Allen et al '077 patent are compared with models of epithermal neutron die-away using exponential terms varying as the sum of detected counting rate components caused by neutrons entering the detector from the wellbore, from the earth formation, and the so-called "thermal background" which results from the detector having at least some residual sensitivity to thermal neutrons. Exponentially weighted moments of the die-away measurements and the model are determined and equated. The equated moments are solved for the ratio of amplitudes of the wellbore component to the formation component. The formation component is determined from weighted moments of the formation and thermal decay components. The determined formation component is used to generated a die-away "constant" which is indicative of the fractional pore volume of the formation. The constant is used in trained neural network computation to generate a neutron porosity corrected for the "standoff" of the detector from the wall of the wellbore.
In the tool of the Allen et al '077 patent the detector is mounted in the pad in order to reduce the effect of irregularities in the surface of the wellbore. Irregularities in the surface of the wellbore can cause error in the epithermal neutron measurements because the wellbore is typically filled with liquid. Liquid in the wellbore slows down the fast neutrons in a very short time and can therefore cause the formation to be irradiated with widely variable numbers of fast neutrons. Subsequent measurements of epithermal neutrons by the detector may be affected by the variations in the original numbers of fast neutrons imparted to the formation.
A drawback to the method and apparatus of the Allen et al '077 patent is that the measurements made by the detector must be compared in a trained neural network to laboratory model measurements in order to generate a porosity measurement which is corrected for the effects of the wellbore, or standoff. If the measurements made by the tool of the Allen et al '077 patent within a particular wellbore should occur outside of the range of the laboratory measurements, then the porosity and standoff values predicted using the neural network can be erroneous. An additional drawback to the method of the Allen et al '077 patent is that the measurements made by the tool are particularly sensitive to the amount of tool standoff. Slight error in determination of standoff can result in significant error in determining the amount of standoff correction to the porosity determination.
It is yet another drawback to the tool disclosed in the Allen et at '077 patent that the porosity measurements made by the tool is subject to relatively large amounts of statistical uncertainty and this uncertainty increases with the amount of standoff. At certain amounts of standoff, the amount of statistical uncertainty can make the measurement relatively difficult to use.
A further drawback to the tool disclosed in the Allen et al '077 patent is that the detector is mounted in the pad on the extensible arm. Construction of an instrument having such an articulated pad can be difficult and expensive, and the measurements made by the tool are still subject to variations in the number of fast neutrons entering the formation since the source is mounted on a substantially centralized tool mandrel. The tool mandrel is subject to variations in the distance between itself and the wellbore wall, which because of the liquid in the wellbore, can cause variation in the number of fast neutrons actually irradiating the formation.
Another epithermal neutron tool is disclosed, for example, in U.S. Pat. No. 5,051,581 issued to Hertzog et al. The tool in the Hertzog et al '581 patent includes a pulsed neutron source and epithermal neutron detectors at axially spaced-apart locations from the source. The epithermal neutron population following the source burst is measured at two of the detectors in order to determine the epithermal neutron slowing-down-length, and the epithermal neutron die-away rate is determined by measurements from the third detector. The slowing down length is relatively insensitive to the effects of the wellbore and tool standoff, and the die-away measurement highly sensitive to the wellbore and standoff, but is only slightly affected by the material composition, called the lithology, of the earth formation. Values of apparent formation porosity for calculated by the slowing-down-length and from the die-away are compared according to an empirical relationship relating apparent porosities to a standoff corrected value of formation porosity.
A drawback to the method and apparatus disclosed in the Hertzog et al '581 patent is that while the slowing-down-length measurement is relatively insensitive to the effects of the wellbore and tool standoff, this measurement is affected by the formation lithology; and the die-away measurement, while relatively insensitive to formation lithology, is still somewhat affected by the lithology. Therefore the tool according to the Hertzog '581 patent provides a formation porosity measurement which is at least partially sensitive to formation lithology. The die-away measurement used in the tool disclosed in the Hertzog et al '581 patent is also subject to relatively large statistical uncertainty, making porosity thus determined less useful than porosity determined by other means.
Accordingly, it is an object of the present invention to provide a method and apparatus for determining the formation porosity using an epithermal neutron tool which is relatively insensitive both to tool standoff and to the formation lithology, and therefore can more easily be corrected for standoff and lithology.